The present invention relates to a bumper beam for automobile that protects an automobile from impact on collision.
One conventional bumper beam for automobile is formed by roll casting. (See patent documents 1, 2 and 3, for example.) FIG. 41 shows a vertical cross-section view of a bumper beam 11 disclosed in patent document 1. The bumper beam 11 comprises a plate material curved by roll-forming, and ends 12 of this plate material are welded to intermediate positions of the plate material. This bumper beam 11 is provided with a rib 15 extending between a front section 13 and a rear section 14. FIG. 42 shows a vertical cross-section view of a bumper beam 21 disclosed in patent document 2. This bumper beam 21 comprises a plate material curved by roll-forming, and end sections 22, 22 are bent and joined so as to be entwined together. FIGS. 43 and 44 show vertical cross-section views of a bumper beam 31 disclosed in patent document 3. This bumper beam 31 comprises a plate material curved by roll-forming, ends 32 of which are inserted into a hole 33 opened in an intermediate position on the plate material and thus joined thereto.
Another bumper beam for automobiles used aluminum extruded materials. For example, as shown in FIG. 45, the bumper beam 41 has a rectangular vertical cross-section with a reinforcement member 42 extending across the center as a cross beam.
Another bumper beam was formed by tube hydro-forming (for example, see Patent Document 4). FIG. 46 is a plan view of this bumper beam 51. The bumper beam 51 is made from a single tubular body. The bumper beam 51 comprises attachment sections 52, 52 protruding toward the body side integrally formed by tube hydro-forming.    Patent document 1: Japanese Patent Application Publication H11-170934    Patent document 2: Japanese Patent Application Publication 2001-260774    Patent document 3: Japanese Patent Application Publication 2002-87186    Patent document 4: Japanese Patent Application Publication. 2003-104140
However, the conventional roll-formed bumper beams 11, 21, 31 or bumper beam 41 that used extruded material, because of their manufacturing method, have a set vertical cross-section shape in the longitudinal direction, and thus have a low degree of freedom of shape. Also, because the bumper beam 51 made by tube hydro-forming comprises a single tubular body, in order to ensure requisite strength, the vertical cross-section shape had to be made large, and reducing size was difficult.
The present invention was conceived in order to resolve the drawbacks of conventional bumper beams, and its object is to provide a bumper beam for automobile that can be made smaller while maintaining a high degree of freedom of shape.